Carbonated specimens after application of the pH indicator solutions

Carbonated specimens after application of the pH indicator solutions (from left, phenolphthalein, thymol blue, alizarine yellow and a mixed indicator)

Source: BAM, division Technology of Construction Materials

The increasing CO2 content in the atmosphere has a direct influence on the durability of concrete based structures. In combination with water, carbonic acid is formed from the CO2 contained in the air. The carbonic acid can react with individual components of the concrete. Since carbon-containing compounds are formed during the reaction, this process is called carbonation. When the cement contained in the concrete sets (hydration), strength-forming hydrate compounds are formed, which cause a high pH value. These hydrate phases are the cause for the passivation of the steel reinforcement often contained in the concrete. Due to the transformation of these hydrate compounds during carbonation, the pH value is lowered and the protective passivation layer dissolves, which can lead to corrosion of the steel and failure of the structure under certain circumstances.

The widely used method to determine the carbonation depth, is the sprayed application of a 1 % phenolphthalein solution (pH-sensitive colourant) on the surfaces of the specimen in accordance to DIN EN 13295:2004-08. However, the use of phenolphthalein is disputed due to the carcinogenic effect of the resulting aerosols. The large-scale use of this dye and the overhead application poses a considerable health risk for the respective user.
The results presented in this paper are an attempt to replace phenolphthalein with the dyes alizarine yellow, thymol blue or a mixed indicator (cresol red and thymol blue). For the evidence of functionality, different concretes were prepared and treated under accelerated carbonation conditions. Subsequently, the standard phenolphthalein test and examinations with the alternative indicators were carried out. In addition, thermogravimetric analysis (TGA), electrolytic back-titration for determination of total CO2 amount and pH analysis of the solubility products were used as reference methods.

The results of the examinations show clearly that the alternative indicators obtain comparable results to the standard phenolphthalein test. Depending on the examined concretes, the results vary, but all show the same tendencies. The readability and the clarity of the results of the alternative indicators are also proved as satisfactory.

Alternative pH-indicators for determination of carbonation depth on cement-based concretes
Nico Vogler, Mathias Lindemann, Philipp Drabetzki, Hans-Carsten Kühne
published in Cement and Concrete Composites, Vol. 109, pages 103565, 2020
BAM, division Technology of Construction Materials